The present invention concerns a process for preparing low molecular weight epoxy resins by reacting an excess of an epihalohydrin with a polyhydric phenol in the presence of a stoichiometric quantity of an alkali metal hydroxide and a halohydrin that is at least as reactive with the alkali metal hydroxide as the halohydrin produced in the reaction and subsequently dehydrohalogenating the resultant product.
In the prior art processes, the reaction to make the bis chlorohydrin has been performed by adding small amounts of sodium hydroxide in increments to a reaction mixture of excess epihalohydrin and bisphenol and when the sodium hydroxide was no longer present or the rate of the reaction became very slow, a further increment of sodium hydroxide would be added. Performing the reaction in this manner, a number of different steps of caustic additions were necessary. The reason for adding small amounts of caustic in this manner is that the sodium hydroxide catalyzes the hydrolysis of epichlorohydrin to glycerine monochlorohydrin.
In order to reduce the rate of the side reaction leading to glycerine monochlorohydrin, which eventually is hydrolyzed to glycidol and glycerine, the prior art has attempted to add the sodium hydroxide in small increments so that the by-product formation from the epichlorohydrin is minimized.
By adding a more reactive chlorohydrin than that which is formed from the reaction if bisphenol A and epichlorohydrin to the reaction mixture at the beginning, it is possible according to the instant invention to add all of the sodium hydroxide at the beginning of the reaction without a consequent loss of epichlorohydrin due to the above side reaction.
One major disadvantage of the prior art processes employing sodium hydroxide as the catalyst is that sequential addition of sodium hydroxide requires either a batch reaction be performed or a series of batch reactions in a continuous manner be performed. By adding all of the sodium hydroxide at the beginning of the reaction, it is possible with the instant invention to feed the mixture simply to a continuous reactor for a single pass through the reactor and achieve high conversions with high yield based on epichlorohydrin.